Apparatus for controlling the apparent resistance of an amplifier anode



Nov. 5, 1940; H. F. MAYER 2,220,770

APPARATUS FOR CONTROLLING THE APPARENT RESISTANCE OF AN AMPLIFIER ANODEFiled Jan. 30, 1937 Inventor's Harry E Mayer,

Patented Nov. 5, 1940 UNITED STATES APPARATUS FOR CONTROLLING THE- AP-PARENT RESISTANCE OF AN AMPLIFIER AN ODE Harry F. Mayer, Schenectady, N.Y., assignor to General Electric Company, a corporation of New YorkApplication January 30,1937, Serial No. 123,213

3 Claims.

My invention relates to amplifier circuits which include an'electrondischarge amplifier. In certain uses of such amplifier circuits it isdesirable that the anode of the amplifier have a resistance which ismaterially different from its natural resistance; for example, in aradio receiver having an audio amplifier feeding a loudspeaker it isdesirable that the resistance of the amplifier anode be low in order toprovide the proper damping for the loudspeaker. In other cases it isdesirable that the anode resistance shall be high, for example, in tunedhigh gain amplifiers where a low anode resistance is undesirable becauseit decreases the gain and the selectivity. Amplifiers with anodes havingthe desired low and high resistances are difiicult and expensive toconstruct if, indeed, they can be produced. 1 g

It is the object of my invention, therefore, to provide improvedapparatus which is connected with the amplifier circuit for causing theamplifier anode, while the circuit is in use, to have an apparentresistance which is different from its actual resistance.

In accordance with my invention I cause the amplifier anode to have anapparent low or high resistance by providing circuit apparatus whichfunctions to supply feedback voltages from the output circuit of theamplifier to the input circuit thereof, the value of one voltage beingcontrolled by the output current and the value of another voltage beingcontrolled by the output voltage. Where a low apparent resistance of theamplifier anode is desired, I make the current controlled feed-backvoltage regenerative and,

the scope of the invention being pointed out in' the appended claims.

Referring to the drawing, Figs. 1, 2, and 3 are circuit diagramsillustrating various embodiments of my invention wherein the amplifieranode is caused to have an apparent low resistance; Fig. 4 is a circuitdiagram illustrating another form of my invention wherein the amplifieris caused to have'a high apparent resistance; and Fig. 5 shows amodified detail of Fig. 4.

In'Fig. 1 where I have chosen to show my invention forming a part of aradio receiving sysv tem of well known form, I is the receiving an- 5tenna, 2 is the radio frequency amplifier stage, 3 is the detectorstage, 4 is the first audio amplifier stage, 5 is the last or poweramplifier stage, and 6 is the loudspeaker. The power amplifier stage 5includes the electron discharge power amplifier i which preferably is apentode having the cathode 8,- the control grid 9, the screen grid 10,the suppressor grid H, and the anode l2. The input circuit of thisamplifier connects with the audio amplifier stage A through thetransformer M and the output circuit of this amplifier connects with theloudspeaker through the load transformer !5.

For the purpose of giving the anode 12 an apparent low resistance inorder better to damp the loudspeaker, I have provided the voltagedivider or potentiometer l'l across the anodecathode circuit of theamplifier I. This potentiometer preferably comprises the resistor l8which may have a resistance, for example, of 10,000 ohms and theresistor 19 which for example may have a resistance of 2,000 ohms whichlatter resistor is engaged by the movable arm across this rheostat,which it will be understood is substantially in phase with the currentin the anode-cathode circuit'and is proportional thereto, is applied tothe primary of the step-up transformer 23, the secondary of which isconnected in a series circuit between the potentiometer arm 20 and thecontrol grid 9 of the amplifier 1 through the secondary of thetransformer M. The secondary of the transformer 23 is so connected inthe series circuit that at the instant at which the cathode end of therheostat the transformer 22 and which is controlled by5 and isproportional to the anode-cathode current is a regenerative voltage; ItWill be seen that the feed-back'voltage which is supplied from thepotentiometer 1'! through the secondary winding of the transformer23'rto the input cir-.-

cuit of the amplifier 7 is a degenerative voltage. With the apparatusdisclosed in Fig. 1, therefore, the apparent resistance of the anode i2is made less than the actual resistance by having 5 a regenerativefeed-back voltage which is proportional to the current in the outputcircuit of the amplifier applied to the input circuit thereof and byhaving a degenerative voltage proportional to the output voltage of theamplifier 10 simultaneously applied to the input circuit thereof By themeans which .I have described above the anode of the amplifier may bemade to have as low a resistance as desired without loss of powersensitivity or increased harmonic distor- 15 tion. The apparent anoderesistance may be reduced to zero or may even be made negative.

In the modified form of my invention illustrated by Fig. 2, I obtaincorrectly phased current-regeneration voltage by the use of a single 20electron discharge amplifier which I use as a resistance coupledamplifier in lieu of the transformer employed in Fig. 1. This modifiedconstruction is of particular value with frequencies of the order of.100 down to 40 cycles where con- 25 siderable phase shift occurs evenwith the best available transformers. In Fig. 2 I obtain thedegenerative feed-back voltage by the same means as shown and describedin connection with Fig. 1, namely the potentiometer H con- 30 nectedacross the output circuit of the amplifier. The regenerative feed-backvoltage which is proportionalto the current in the output circuit of theamplifier is obtained by connecting the input circuit of the electrondischarge amplifier 35 25 across the resistor 21 and connecting theoutput circuit of the amplifier 26, which includes the arm 25) of thepotentiometer, with the input circuit of the amplifier l, a suitableblocking capacitor 28 being employed [to keep the direct current off thegrid of that amplifier. In this figure the amplifier 26 is so shown thatthe phase of the current controlled feed-back voltage is properly phasedto be regenerative when applied to the grid of the amplifier In themodification illustrated by Fig. 3 I have shown how the preceding audioamplifier stage may be employed in lieu of the separate amplifier 26 toreverse the phase of the current responsive feed-back voltage. In thiscase the po- ,50 tential drop across the resistor 39 in the anodecathodecircuit is applied to the input circuit of the amplifier 3| comprisingthe preceding audio amplifier stage, the output circuit of whichamplifier connects in the usual manner through 55 the capacitor 32 withthe input circuit of the amplifier l. A voltage responsive degenerativefeed-back voltage in this case is obtained by the voltage divider orpotentiometer 33 which connects across the primary of the loadtransformer 0 I5. As will be seen from Fig. 3, the potentiometer arm 34connects through the resistor 35 constituting the principal load on theamplifier 3| with the input circuit of the amplifier 1. In certain casesit may be found desirable to shunt the .65 resistor 30 with a capacitoror with a capacitor in series with a resistor in order to compensate forthe internal impedance of the power supply, as

is obvious.

In the modification illustrated by Fig. 4 I have shown how the amplifieranode may be made to have an apparent resistance which is greater thanthe actual resistance thereof. This is done by supplying to the inputcircuit of the amplifier simultaneously a degenerative feed-back voltage.75 which is proportional to the current in the output circuit of theamplifier and a regenerative feed-back voltage which is proportional tothe voltage of the output circuit of the amplifier. The amplifier inthis case is illustrated as comprising a stage of intermediate frequencyamplification and is represented at 7. The radio receiving systemillustrated by Fig. 4 includes the converter stage 38, the intermediatefrequency transformer 39 connecting stage 38 with the amplifier 1, theintermediate frequency transformer 40 in the output circuit of amplifierl, the second intermediate frequency amplifier stage 4|, the

second detector stage 42, the first audio amplifier stage 43, the secondaudio amplifier stage 44, and the loud-speaker 6. The transformer 40 isprovided with the auxiliary or tertiary winding 45 which is wound in thesame direction as, ooaxially with, and close to the primary winding ofthis transformer. Winding 45 connects in series with the cathoderesistor 46 between ground and the cathode of the amplifier I. Thesuppressor grid connects with the ground through the by-pass capacitor477 and with the cathode through the high resistor 48. In this form ofmy invention a degenerative voltage, which is the voltage drop acrossthe resistor 45, is supplied to the input circuit of the amplifier I,which voltage is proportional to the current in the output circuit ofthe amplifier. Also, a regenerative voltage is applied to the inputcircuit of the amplifier I, which voltage is induced in the winding 45and which is proportional to the alternating voltage applied to theprimary of the transformer 4!). In that form of my invention shown byFig. 4, therefore, the anode of the amplifier is caused to have anapparent high resistance as it is seen from the load by supplying to theinput circuit of the amplifier a degenerative feed-back voltage which isproportional to the current in the output circuit of the amplifier, anda regenerative 40 voltage which is proportional to the voltage of theoutput circuit thereof. The modification illustrated by Fig. 4 isdisclosed and claimed in my divisional application Serial No. 316,953,February 2, 1940, entitled Apparatus for controlling the apparentresistance of an amplifier anode.

In certain cases it may be desirable to reduce the amount of directcurrent cathode bias produced by the resistor 46 of Fig. 4. This may bedone in the manner shown by Fig. 5 where the resistor 46 is replaced bytwo separate resistors 48 and 49 and the latter is connected across aresonant circuit comprising the capacitor 5i) and the reactor 5|. Thiscircuit being resonant at signal frequency offers a high impedance tothe signal frequency in the cathode circuit, but the reactor element 5|thereof readily passes direct current.

I have chosen the particular embodiment described above as illustrativeof my invention and it will be apparent that various other modificationsmay be made without departing from the spirit and scope of my inventionwhich modifications I aim to cover by the appended claims.

What I claim as new and desire to secure by 6 Letters Patent of theUnited States, is:

1. In combination with an amplifier circuit including a first and asecond electron discharge amplifier connected together in cascade forthe transmission of signals and said second amplifier having an anode,feed-back apparatus for causing said anode to have a lower apparentresistance comprising means for supplying a degenerative voltage to theinput circuit of said second amplifier controlled by the voltage of the'15 output circuit thereof and means for supplying regenerative voltageto the input circuit of said first amplifier controlled by the currentin the output circuit of said second amplifier.

2. In combination with an amplifier circuit including a first and asecond electron discharge amplifier connected together in cascade forthe transmission of signals and said second amplifier having an anode,feed-back apparatus for causing said anode to have a lower apparentresistance comprising a load device in the output circuit of said secondamplifier, a resistor across said device, means for supplying thepotential drop across a portion of said resistor degeneratively to theinput circuit of said second amplifier and means for supplying aregenerative voltage to the input circuit of said first amplifier inresponse to the current in the output circuit of said second amplifier.

3. In combination with an amplifiercircuit including a first and asecond electron discharge amplifier connected together in cascade forthe transmission of signals and said second amplifier having an anode,feed-back apparatus for causing said anode to have a lower apparentresistance comprising a load device in the anode circuit of said secondamplifier, a potentiometer in shunt with said device and having its armconnected to supply a degenerative voltage to the input circuit of saidsecond amplifier, a resistorin the cathode circuit of said secondamplifier and means for supplying the voltage drop in said resistor as aregenerative voltage to the input circuit of said first amplifier.

HARRY F. MAYER.

